The party is venturing in a ruined castle when they noticed in the corridor in front of them a door is ajar, and a sliver of flickering light, probably from a torch, comes out of it. As they get close, they can hear faint voices, people discussing something.
The heroes reach the door and peek inside, cautiously: Two men stand in the room, in front of a large wooden chest sitting in the middle. One of the man leans against a wall with a torch in his hand and an angered expression on his face. The other is kneeling in front of the chest, giving his shoulders to the door. They both seem dressed like common adventurers or explorers, and decently armed.
"Don't do it, Jeff, you know these places are full of traps." Says the first man.
"Stop worrying, I checked, there is nothing-" Before the second man can finish, the wooden chest springs open, revealing a hideous mouth and a long tongue. The man has barely time to scream before the horrible creature has engulfed him, only his legs peek out of the monster, swinging wildly. Blood drips all over.
The first men yells, pulls his sword and jumps to attack the monster, when the heroes, burst into the room, ready to fight with him.
In an instant they have surrounded the beast, the unknown adventurer is about to strike first, He lifts his sword, and swings.
At one of the heroes.
At the same time, the door slams closed, and a set of teeth appears around it from the walls. A foul-smelling, thick liquid oozes from floor and walls. Before the group has time to realize what is going on, the chest hurls itself at them, with the man still trapped inside.
For in truth, there never was a man. Or a chest. The entire room, including the mimic and the two adventurers in it, are fake, and part of the actual mimic that forms the entire room and the scenario taking place in it. And the heroes have already fallen for its devious trick, now trapped inside him.
Many adventurers will expect danger from abandoned chests, the most seasoned will be wary even of a simple room or corridor, and always be on the lookout for traps and tricks. But there is one moment where almost nobody will be on his guard, when nobody fully expect a mimic: when the fight with the mimic has already started.
Nobody goes to look for a mimic when one is already out. Their attention drawn to the fake mimic, forced by the situation to hurry, they'll be unprepared for the real trap.
The mimic room mimic is a very large, powerful and smart mimic that creates an enticing scenario of people being attacked by a mimic to lower the guard of its victims as much as possible, in a matrioshka of mimicry.
The mimic attaches itself to the dungeon structure and grows to fill an entire room, replacing only the entrance door completely, so that leaving it once it has attacked is very hard.
The mimic is very clever, it can use vibrations in the walls and floors he's attacked to to perceive sounds, learn the position of the party and start its scenario just so that they reach it at the pivotal moment.
If the adventurers were to look for it, they could notice the inside of the room doesn't look quite right, at a deeper observation: the texture of the objects in it isn't quite perfect, and it's very noticeable if touched. It also exudes its digestive juices at all times, even if in small and hard to notice quantities, they can be smelled or seen before the fight starts. In any case the room inside and outside is dimly lit, making all of that hard to notice.
Spells that look for living creatures will obviously reveal the trap, and so will a deep examination of the entrance door: The mimic relies on the adventurers being too busy to pay attention, its fine details aren't quite perfect.
Once the fight started, the mimic will attack with pseudopods trying to bash the heroes. The two fake adventurers and the fake mimic are in fact nothing more than that, fleshy extensions of the mimic, and they are pseudopods in themselves. Their swords not real metal. A very careful observer will notice the feet of the two are, very loosely, attacked to the floor with a fleshy filament under their boots. The room can spawn other, normal looking pseudopods to fight other adventurers if 3 aren't enough.
The room also exudes acidic juices that can drip on people and do acid damage, they also rapidly coat the walls causing damage to any that touches them. In a few minutes, the room runs out of air and is filled with toxic gas.
The door to the fake room turns into a jaw, and it's firmly locked, it is also made of flesh material and not wood making it much harder to break through, it is also coated in acid as well.
Once the room has killed its victims, it will take a few days digesting them, any gear that can't be digested will be attacked to the fake adventurers to make them look more realistic for future traps. While digesting, the mimic is weakened and drowsy and the trap will not happen. The door will simply change shape to look like a section of wall, hiding the mimic room mimic.
